The present invention relates to a local area network (LAN) constituted such that nodes each including at least one data terminal and a network interface unit are connected to networks having different characteristics, wherein a message can be transferred at high speed between the data terminals by means of packet switching, in accordance with a message size.
Computer systems and various other office automation (OA) apparatuses are very popular to improve working efficiency in offices and effectively utilize data and the like. At the initial stage of development, such apparatuses are used as stand-alone equipment at proper locations. However, this configuration does not allow apparatuses to be systematically connected and to be maximally used. Therefore, a local area network has received a great deal of attention as a system for connecting the apparatuses.
In a broad sense, the LAN system is a network in which computers, large-capacity memories, printers, monitor equipment, and control equipment, all being distributed in a relatively narrow area, are connected through transmission lines. In a narrow sense, the LAN system is a digital information communication network in an enterprise. FIG. 1 shows a conventional network of LAN system 6.
As is shown in FIG. 1, LAN system 6 comprises network manager NM, high-speed data transfer network 8, and control data transmission network 7, which is used to transmit data or control data between nodes N1 to Nn constituted by computers or the like within buildings. Each of nodes N1 to Nn comprises at least one data terminal and one network interface unit NIU and performs data communication by means of packet switching system, i.e., one of the store and switching schemes. When a channel establishment request from a given data terminal is output through network interface unit NIU, a network manager NM establishes a channel in accordance with channel establishment requests from other data terminals. Network 7 is used to transmit control data representing the number of packets and the number of idle packets, and comprises a data transmission medium (e.g., coaxial cables or low-speed transmission optical fibers) for connecting nodes N1 to Nn. Network 8 is used to transmit a data packet, and comprises high-speed transmission optical fiber cables radially connected to nodes N1 to Nn with respect to star coupler 9 as the center of the network.
Each message is divided into data blocks, each having a length of a plurality of bytes, and is transmitted to a destination terminal in units of packets affixed with a destination number or the like. As is shown in FIG. 2, when a long message, such as X-ray image data, is to be transmitted from terminal 2-1 to terminal 2-2, control data is generated before and after the image data is transmitted from terminal 2-1. NIU 4-1 connected to terminal 2-1 determines whether the series of input transmission data is control data or image data. When the transmission data is control data, it is transmitted to network manager NM through network 7, thereby establishing a channel within network 8. When the channel is established in network 8, NIU 4-1 transmits image data to the destination terminal through network 8. When image data transmission is completed, the control data is transmitted again from terminal 2-1 to terminal 2-2 through NIUs 4-1 and 4-2. Thus, transmission of the image data in one execution cycle is completed.
In the case above mentioned, a data transmission rate is important. The conventional network system shown in FIG. 1 includes high-speed network 8 having a high transmission rate and low-speed network 7 having a low transmission rate. High- and low-speed networks 8 and 7 employ different protocols. When network 8 is used with a small data capacity, the data can be transmitted at high speed, as shown in FIG. 3A, but transmission time T2 including a setup time is long. When network 7 is used, as is shown in FIG. 3B, a long time is required to prepare and transmit the data, but transmission time T1 itself is short. Therefore, when a small amount of data is to be transmitted, protocol processing time T1 is shorter than protocol processing time T2.
When a large amount of data is to be transmitted, transmission time T4 required for use of network 8 is substantially the same as transmission time T2 required to transmit a small amount of data through network 8, as is shown in FIG. 3C. When a large amount of data is divided into units of a smaller units of data, and when these data units are transmitted through network 7, as is shown in FIG. 3D, transmission time T3 is considerably longer than transmission time T4, as is evident from FIG. 3C.
As described above, when the data is transmitted through network 8 regardless of the length of the transmission data, the control data transmission time is longer than the message data transmission time if the message transmission data is short. Therefore, networks 7 and 8 cannot be effectively utilized. For this reason, in a network for transmitting both large and small data blocks, e.g., in a medical image data storage communication system, transmission efficiency for transmitting a small data block is low in a high-speed network. And, transmission efficiency for transmitting a large data block is low in a low-speed network. A strong demand, therefore, has arisen for developing a LAN system capable of effectively transmitting data, on the basis of message sizes, through a plurality of networks having different data transmission rate characteristics.